8 Medical-Related Reasons for Unexplained Weight Gain

Eating healthy? ✔️ Working out? ✔️ Still struggling with unexplained weight gain? ✔️ One of these medical problems might be the reason.

Fact: Weight fluctuates. Maybe you lost a few pounds toward the end of graduate school — after all, the stress of, say, completing a thesis can diminish most appetites. Or perhaps you put on a few pounds last holiday season after a month-plus of celebrations (latkes! Christmas cookies! champagne!) and fewer workouts. Eh, it happens.

Fluctuating weight alone is not a problem — in fact, it's normal. But if you're eating healthy and exercising per usual and the number on your scale keeps going up, then that doesn't add up. Gained 20 pounds in a month? Now that's cause for concern.

This kind of totally unexplained weight gain can signal more serious health problems such as a hormone condition or a reaction to an Rx. Ahead, a few potential medical reasons for unexplained weight gain. (Remember, though: The only way to really know the cause of your sudden weight gain is to consult your doctor.)

Polycystic Ovary Syndrome

Affecting 1 in 10 people with ovaries of childbearing age, polycystic ovary syndrome (PCOS) is a hormonal disorder that can impact or even inhibit fertility and cause a range of other health issues, including excess facial and body hair and unexplained weight gain, according to the Office on Women's Health (OWH). The exact cause of PCOS is unknown, but most experts believe it occurs when the adrenal glands pump out too much of the "male hormones" or androgens (such as testosterone), which can stop ovulation and cause irregular periods — two of the hallmark signs of PCOS.

It's not the extra androgens that trigger the seemingly unexplained weight gain, though. It's actually "multiple factors, but most notably insulin resistance," says Minisha Sood, M.D., a board-certified endocrinologist in New York City. People with PCOS are often insulin resistant, meaning their bodies can make insulin (a hormone that controls how the food you eat is changed into energy) but can't use it effectively. "In someone with insulin resistance, it takes more insulin to store glucose (or blood sugar) in the body's cells than under insulin-sensitive conditions," explains Dr. Sood. "And high levels of insulin can promote weight gain," she says. (See also: The Connection Between PCOS and IBS)

Unfortunately, there's no cure for PCOS, but there are a variety of lifestyle changes and medications that can help you manage symptoms. If your doctor determines — through different tests and a physical exam — that PCOS is a reason for your unexplained weight gain, they'll likely discuss treatment methods such as diet, exercise, and "sometimes medications, such as metformin, that make the body more sensitive to insulin," says Dr. Sood.

Water Retention

Extra pounds don't always equal fat but are sometimes due to fluid retention — familiar to most people with premenstrual symptoms. If the unexplained weight gain isn't related to the menstrual cycle, though, it shouldn't be ignored. "If someone's retaining a lot of water — enough to add more than a couple of pounds — they'd better get to their physician very quickly to make sure they don't have heart or kidney failure, both of which can cause edema or swelling," cautions Robert Berkowitz, M.D., medical director of the University of Pennsylvania Weight and Eating Disorders Program. Though, such problems are much more likely to afflict older women, he adds. "If you push a fingertip into your skin and it leaves a real indentation rather than springing back, that's a tip-off that it's fluid, not fat," he notes.

Other symptoms of these serious medical conditions include shortness of breath (congestive heart disease), decreased urine output, and loss of appetite (kidney failure), as well as fatigue and increased abdominal girth even without weight gain (heart and kidney issues). Liver disease and certain cancers can cause abnormal fluid accumulation in the abdomen as well, so any big boost in your waist size, with or without unexplained weight gain, warrants a look by your doctor, advises Howard Eisenson, M.D., a family medicine physician in Durham, North Carolina. (BTW, here are six other sneaky reasons you're not losing weight.)


Essentially an underactive thyroid, hypothyroidism occurs when your thyroid doesn't make enough thyroid hormones, thereby slowing down many of your body's functions, such as your metabolism, according to the OWH. This, in turn, can lead to a greater "retention of water and salt," which is typically to blame for the small increase on the scale, says Dr. Sood. Another reason unexplained weight gain can also occur in hypothyroidism is that "the resting energy expenditure or the amount of energy the body burns in the form of calories can decrease," she adds. (More: I'm a Fitness Influencer with an Invisible Illness That Causes Me to Gain Weight)

While hypothyroidism can definitely be a reason for unexplained weight gain, it's mostly "mild weight gain," or roughly 5 to 10 pounds, says Dr. Sood. If you're experiencing other telltale symptoms such as brittle hair and nails, dry skin, and a tendency to feel cold, then definitely visit your doc, who can determine if hypothyroid is truly a reason for your sudden weight gain through a blood test.

In addition to lifestyle changes (such as lowering intake of processed foods and sugars or prioritizing physical activity), hypothyroidism can be treated using medication "to replace the amount of thyroid hormone that the affected person is no longer able to produce," explains Dr. Sood.


If you've ever spent an evening tossing and turning — or maybe sans sleep altogether — you know what's in store for the following day: cravings, and lots of them. That's because not getting enough sleep can increase your levels of ghrelin (the hormone that signals when it's time to eat) and decrease your levels of leptin (the hormone that tells your body when it's full), according to the National Institute of Health's National Heart, Lung, and Blood Institute. And thanks to this imbalance of hormones, you're left feeling hungrier than you would when well-rested and, in turn, likely snacking more, too — after all, lack of sleep can also impact your decision-making. (See also: How to Balance Out-of-Whack Hormones)

What's more, "people who go to bed later or cannot sleep may eat more calories overall because of the late-night eating," says Wendy Bennett, M.D., M.P.H., associate professor of medicine at the Johns Hopkins University School of Medicine and Bloomberg School of Public Health. So, if you've been struggling with unexplained weight gain, take a second to think about your sleep habits and potential ways to score some more shut-eye.

Cushing's Disease

When it comes to potential reasons for unexplained weight gain, Cushing's disease might be at fault, but the condition is rare, affecting about 40 to 70 people per million. Cushing's disease occurs when your body produces too much of the stress hormone cortisol, as a result of long-term steroid use or a tumor on the pituitary gland in the brain. "This tumor overproduces a hormone called ACTH, which then stimulates the adrenal glands to overproduce cortisol," explains Flora Sinha, M.D., a primary care physician at Cedars-Sinai in Los Angeles. "This excess cortisol causes very distinct symptoms of fullness of the face, fat deposits over the upper back, excessive fat over the abdomen along with weakness, skin changes, easy bruising, high blood pressure, and more," she explains. (More: The Link Between Cortisol and Exercise)

The serious illness might also cause purplish stretch marks on the abdomen and excess hair growth on the face, neck, chest, abdomen, and thighs, according to the National Institute of Health's National Institute of Diabetes and Digestive and Kidney Diseases. If you experience any of these symptoms (in addition to your unexplained weight gain), be sure to schedule a visit with your doctor, stat. If they suspect the disease, your doc will likely measure certain hormone levels in your blood, urine, and/or saliva, says Dr. Sinah. "Treatment options are individualized to patients and may include surgery, medication, radiation therapy," she adds.

Mood Disorders

Mood disorders, including stress, anxiety, and depression, all have a "complex relationship" with weight gain, according to Avi Varma, M.D., a family medicine physician in Atlanta. "It is unclear if weight gain can lead to mood disorders or vice versa, but the likely answer is that it can occur both ways," she says.

Think of it this way: When you're feeling down or super stressed, you might turn to food for comfort, which can ultimately lead to seemingly unexplained weight gain. When dealing with depression or anxiety, it's also possible to feel somewhat stuck — as if you'd rather just lay in bed than do anything involving movement. And while that's completely understandable (and seeing an expert can help), "maintaining a sedentary lifestyle in a depressed state can also trigger weight gain due to inactivity," adds Dr. Varma. And then there's chronic stress, which in addition to influencing your diet and lifestyle choices, can send your body into a prolonged state of fight or flight, increasing cortisol levels and "causing the body to hang onto calories and gain weight," says Dr. Bennett.

Bottom line: "Mood disorders can impact patients' sleep patterns, activity levels, and appetite, which, in turn, affects their weight and how they retain fat," notes Dr. Sinha. Seeing a therapist can help you untangle your emotions and, in turn, deal with any unexplained weight gain. (More: Why Everyone Should Try Therapy at Least Once)


Do you take any medicines on a regular basis? Then there's a chance that one of them may be the cause of your unexplained weight gain. Some medicines that can cause sudden weight gain are:


Of the widely prescribed SSRIs (selective serotonin reuptake inhibitors) and SNRIs (selective serotonin and norepinephrine reuptake inhibitors), many researchers and clinicians believe that paroxetine tends to produce the most weight gain, though typically not more than several pounds. Other antidepressants, such as fluoxetine, sertraline, and venlafaxine also may be to blame for unexplained weight gain, especially if used long-term. (See also: The Stigma Around Psychiatric Medication Is Forcing People to Suffer In Silence)

One of the most popular mood stabilizers is bupropion, which, if anything, often helps patients lose a few pounds. The point is that within each class of anti-depressants are some that tend to produce more weight gain and some that tend to produce less, but at the end of the day, it's most important to weigh the pros and cons based on the patient, says Dr. Varma. "If the antidepressant helps to improve an individual's mood and allows them to leave their sedentary lifestyle to become more active and to eat healthily, it may be best to keep the person on the antidepressant," she notes.

Anti-Diabetes Drugs

Ironically, medications for type II diabetes — in which your body either resists the effects of insulin or doesn't produce enough insulin, according to the Mayo Clinic — are often responsible for further weight gain. And given that one of the key treatments for type II diabetes tends to be weight loss, this side effect creates a vicious cycle. Although it's not suitable for everyone, one effective anti-diabetes drug called Glucophage (aka metformin, which is also helpful for PCOS) does not increase weight, says Dr. Berkowitz. And even some patients on other anti-diabetes drugs may be able to lose weight, under their doctor's supervision, by scaling back on their dosage or adding a weight-loss drug such as orlistat, which partially blocks fat absorption, he says. But you'll want to consult your doctor about the best option for you.

Oral Contraceptives

Hormonal birth control has long gotten a bad rep for causing (seemingly) unexplained weight gain. That doesn't mean it will necessarily cause the scale to creep upward, but if you are particularly concerned about potential weight gain or have experienced this side effect already, chat with your doc about other options. The low-dose pills commonly prescribed today shouldn't add more than a few extra pounds, according to Dr. Berkowitz. Some sources note that there is less of a connection between certain birth control and weight gain than previously thought.


The most commonly prescribed steroids are called adrenocorticoids and are used to control severe autoimmune problems, including asthma, arthritis, lupus, and inflammatory bowel disease. Long-term use can increase appetite and increase weight by 20 pounds or more, says Dr. Berkowitz. But because the symptoms these steroids alleviate are potentially life-threatening, you don't have much choice but to be on them when necessary. However, doctors should be vigilant about cycling patients off medication when they don't need it, which can help them lose some of the weight they may have accumulated, he adds. (See also: What You Need to Know About Getting a Cortisone Shot)

"Many people who are on a medication and start to gain weight simply stop taking their medicine. Don't do that!" cautions Dr. Berkowitz. "Keep taking it while you ask your doctor about switching to something else," he adds.


While unlikely, it's worth mentioning that "if women have rapid, unexplained weight gain, it's possible, though rare, that they have a tumor," says Dr. Eisenson. One example: ovarian tumors — some of which are benign — such as a dermoid tumor, a weird conglomeration of various body tissues (sometimes including teeth) that grow in the abdomen. "We've had patients gain more than 100 pounds because of a huge ovarian tumor in their belly," he adds. Again, don't ignore any disproportionate expansion of your mid-section — check in with a physician.

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